HCl is present as an impurity in numerous industrial fluids, i.e. liquid and gas streams. For example, in reforming processes conducted in refineries and petrochemical plants, a chlorine promoted catalyst is generally employed. During the reforming operation, which also generates hydrogen, a small amount of gaseous HCl is produced which is carried away with the product streams, hydrogen-rich gas being one of the streams. The hydrogen containing the HCl is either recycled to the reformer or used in other processing units in which hydrogen is required as a reactant. Accordingly, the small amount of gaseous HCl, an acid gas, present in the hydrogen can seriously interfere with the operation of the process and, in addition, can cause acid-related corrosion and fouling problems. Additionally, there are other processes in which small amounts of HCl are generated and carried away in gas or liquid streams and which must be removed from such streams to prevent corrosion damage to equipment and avoid environmental problems.
It is well known that activated alumina can act as a scavenger for the removal of small quantities of HCl from fluid streams by taking advantage of the inherent physical attraction between activated alumina and HCl (physical adsorption). Typically, HCl scavengers made from alumina are formed into nodules, e.g., spheres, the spheres forming a fixed bed through which the gas to be scavenged flows. Handling and use of the nodules dictates that they possess sufficient crush strength to retain structural integrity.
Because the adsorption capacity of activated alumina is somewhat limited, a number of researchers have investigated methods of enhancing its performance. For example, U.S. Pat. No. 4,762,537 (Fleming et al.) teaches an HCl adsorbent comprising a major amount of activated alumina and a minor amount of acid-treated Y zeolite. Also, U.S. Pat. No. 4,639,259 (Pearson) teaches an HCl adsorbent comprising activated alumina impregnated with an alkaline earth metal salt. Although both of these patents teach enhanced activated alumina adsorbents possessing improved HCl adsorption capacities, neither discusses whether their adsorbent nodules maintain sufficient crush strength (about 15 pounds) to be useful in industrial applications.
The maintenance in promoted activated alumina adsorbents of sufficient nodule crush strength for practical industrial application has been studied by a number of researchers. It has been generally recognized that the HCl adsorption capacity of activated alumina can be increased through the impregnation of the alumina with alkali oxide, but that the percentage of alkali oxide on the finished adsorbent must be limited to avoid reducing the crush strength of the finished adsorbent nodules to below levels acceptable for practical industrial applications. Pearson, the inventor in U.S. Pat. No. 4,63 9,259, later collaborated with Lee in U.S. Pat. No. 5,505,926 (Lee et al.) and U.S. Pat. No. 5,316,998 (Lee et al.) to teach an HCl adsorbent comprising activated alumina impregnated with more than 5 wt. % alkali metal oxide, wherein the crush strength of the finished adsorbent nodules can be maintained at high enough levels for practical industrial application through the use of a water soluble, alkali metal salt of a carboxylic acid containing from 1 to 6 carbon atoms as the promoter precursor.
The incorporation of a metal oxide base in the activated alumina is especially desirable since it means that the user can enjoy longer run times before having to replace the adsorbent, or in the case of new units, can design the units smaller. By increasing the content of promoters such as sodium oxide or calcium oxide, the HCl adsorption capacity of activated alumina can be increased. The improved hydrogen chloride adsorption capacity of alumina impregnated with alkaline or alkaline earth oxides (e.g., Na.sub.2 O, CaO, MgO) is attributable to the addition of a second mechanism of HCl adsorption based upon simple acid-base chemistry. The weakly basic metal oxide functions to neutralize the strongly acidic HCl in the process fluid, resulting in the by-products of water and alkali or alkaline earth chloride salt: EQU 2HCl+M.sub.2 O.fwdarw.2MCl+H.sub.2 O
M=Li, Na, K EQU 2HCl+MO.fwdarw.MCl.sub.2 +H.sub.2 O
M=Be, Mg, Ca
Still other researchers have studied the use of amine-impregnated materials as agents for the removal of contaminants from refinery and chemical plant streams. U.S. Pat. No. 3,491,031 (Stoneburner) teaches the use of amine-treated activated carbon to remove acid gases, primarily CO.sub.2 but also NO.sub.2, SO.sub.2, H.sub.2 S, HCN, SO.sub.3 and CS.sub.2, from refinery gas streams with MEA being the amine of choice. U.S. Pat. No. 4,531,953 (Groose et al.) teaches removal of toxic gases such as cyanogen chloride from a gas stream with amine-treated activated carbon. And U.S. Pat. No. 5,616,533 (Tavlarides et al.) teaches the use of ceramic (e.g. alumina) to remove metals from liquid streams, wherein one embodiment comprises covalent attachment of thio and amino groups on alumina.
Although activated alumina has been used to scavenge HCl from refinery streams, its use in these services also catalyzes certain side-reactions that are detrimental to refinery operations. Both non-promoted and metal oxide-promoted activated aluminas display these catalytic tendencies to varying degrees. Activated alumina adsorbents tend to promote the polymerization of olefins due to their inherent acidity or resultant acidity from use in such service. Since olefins are commonly present in these applications, the resultant polymer affords a serious deposition and fouling concern to downstream equipment. This chemical reactivity is known in the industry as "Green Oil" formation: EQU nR.sub.2 C.dbd.CR.sub.2 +H.sup.+.fwdarw.--(R.sub.2 C--CR.sub.2).sub.n --("Green Oil")
An adsorbent that offers superior HCl-scavenging capacity with reduced tendency to catalyze the formation of Green Oil would be of value to the refining and chemical industries.